The present invention relates to an information display medium.
It is known in the art that visual information is written on the surface of a display medium such as a film, and the display medium is exposed for display through a display window of a display device.
One display medium used in such an application is a thermosensitive display medium which has a reversible effect such that it can be colored when heated and become colorless upon being cooled, the display medium being made of a chemical material exhibiting hysteresis with respect to temperature. This display medium displays image information based on the phenomenon that two conditions, i.e., colored and colorless conditions can exist simultaneously at a certain temperature. In order to keep image information displayed on the display medium, the display medium has to be kept at such a temperature by being heated or cooled, for example.
Recently, there has been found a thermosensitive material which can reversibly repeat turbid and transparent conditions by being heated to different temperatures and can keep one of such conditions stably below a particular temperature.
This thermosensitive material (hereinafter referred to as a "thermochromic material") is made of a matrix material such as thermoplastic resin and an organic low-molecular material dispersed in the matrix material. If the thermochromic material is heated to a temperature above a particular temperature t.sub.0, the condition of the thermochromic material after it has been cooled below t.sub.0 varies dependent on the temperature to which the material was heated.
More specifically, as shown in FIG. 7, the thermochromic material has two condition transition areas t.sub.0 -t.sub.1, t.sub.1 -t.sub.2 (t.sub.0 &lt;t.sub.1, t.sub.1 -t.sub.2) above t.sub.0. If the thermochromic material is heated above t.sub.2 and then cooled below t.sub.0, then the material becomes turbid, cutting off light.
If the thermochromic material is heated to the temperature range indicated by t.sub.1 and then cooled below t.sub.0, then the material becomes transparent with high transmissivity.
In case the thermochromic material is heated to a temperature between t.sub.0 -t.sub.1 or t.sub.1 -t.sub.2, the turbidity of the thermochromic material upon being cooled ranges continuously from a transparent state to a fully turbid state dependent on the temperature to which the material was heated.
In FIG. 7, the solid-line curve represents the light transmissivity of a thermochromic material while being heated which is initially in a turbid state, and the broken-line curve indicates the light transmissivity thereof after being cooled below t.sub.0. A thermochromic material which is in a transparent state exhibits the same characteristics except for different transmissivity below t.sub.1.
Examples of the thermochromic material are as follows: The matrix material may be one or more of thermoplastic resins such as polyester, polyamide, polyacrylate, polymethacrylate, polystyrene, silicone, polyvinyl chloride, copolymer of vinylidene chloride and vinyl chloride, copolymer of vinylidene chloride and acrylonitrile, and the like.
The organic low-molecular material may be one or more of organic compounds having 10 to 30 carbon atoms, such as saturated or unsaturated monocarboxylic and dicarboxylic acids or their ester, amide and ammonium salt, saturated or unsaturated halogenated fatty acid or its ester, amide and ammonium salt, and aryl carboxylic acid or its ester, amide and ammonium salt. Particularly preferable are linear saturated higher fatty acids such as palmitic acid, stearic acid, arachic acid, and behenic acid, or their ester, amide and ammonium salt.
The thermochromic material represented by the above substances is deposited on a colorless transparent substrate to provide a partial turbid state, and this assembly is constructed as one information display medium.
The assembly is used as one unit, and such assemblies are combined in layers which may be combined with a colored background panel, for example, for displaying image information (see Japanese Laid-Open Publications Nos. 57-92370 and 57-212481, for example).
Where the substrate or the thermochromic material is colored for displaying image information, colored display is made possible. The substrate or the thermochromic material is often colored in a single color. Any display obtained is monotonous since image information cannot be displayed in a combination of different colors based on the property of the thermochromic material.